The present invention generally pertains to electronic distance measurement techniques, and is particularly directed to systems and methods for determining the range from a vehicle, such as a ship, to a plurality of reference points.
A prior art system of this nature is described in U.S. Pat. No. 4,011,562 to Robert L. Bruce. A generalized schematic representation of such system is shown in FIG. 1; and the waveforms of the various signal pulses communicated within the system of FIG. 1 are shown in FIG. 2. The system basically includes a mobile transceiver 10, which is located on the vehicle, a plurality of reference transponders 11, 12, 13, 14 respectively located at a plurality of reference points 1,2,3 and 4, a data processor 15 that is coupled to the mobile transceiver 10, and a reference transmitter 18. The locations of the reference points are presumed to be known. Typically the reference transmitter 18 is located at a reference point. The reference transmitter 18 transmits a timing pulse TP. The mobile transceiver 10 receives and responds to the timing pulse TP by interrogating the plurality of reference transponders 11, 12, 13, 14 to enable the data processor 15 to determine the range from the vehicle to each of the reference transponders. The mobile transceiver 10 transmits a multiplexed sequence of ranging interrogation signal pulses I1, I2, I3 and I4 that are modulated on an RF carrier having a given frequency. The individual reference transponders 11, 12, 13, 14 respectively respond to the timing pulse and the separate interrogation signal pulses I1, I2, I3, I4, as shown in FIG. 2, by transmitting ranging response signal pulses R1, R2, R3 and R4 on a carrier having the given frequency. Within each time slot as determined in relation to the timing pulse, there is a sequence of the respective pairs of interrogation signal pulses and response signal pulses, to wit: I1, R1, I2, R2, I3, R3, I4, R4. In FIG. 2, a transmitted signal pulse is shown by solid lines and a received signal pulse is shown by dashed lines.
In responding to the timing pulse and a detected ranging interrogation signal pulse "I", a reference transponder 11, 12, 13, 14 measures the received carrier phase of the interrogation signal in relation to an internally generated reference signal and later transmits a ranging response signal pulse "R" which replicates the carrier frequency and phase. The ranging response signal pulses R1, R2, R3, R4 are delayed in time from the end of the respective interrogation signals I1, I2, I3, I4 to which they respond, but they are phase coherent therewith. The ranging response signal pulses R1, R2, R3, R4 are received by the mobile transceiver 10. The data processor 15 then determines the range from the vehicle to the reference point at which the reference transponder is located by determining and processing the phase of the ranging response signal pulse received by the mobile transceiver 10 in relation to the transmitted ranging interrogation signal pulse. Upon determining the range from the vehicle to the plurality of reference points 1,2,3,4, it is then possible to determine the position of the vehicle by multilateration.
All phase-comparison electronic distance measuring systems have range ambiguities at multiples of the half-wavelength of the RF carrier. To resolve the ambiguities, generally one or more additional carriers whose frequency is a fraction (and wavelength a multiple) of the RF carrier having the given frequency are used to resolve the ambiguity of the range measurement.
Sometimes an offset carrier signal at a second frequency is added to or modulated onto the RF carrier having the given frequency, the offset being equal to the desired lower frequency. Generally, however, the second frequency carrier signal is transmitted simultaneously. Accordingly the frequency channel over which the transmissions are carried must be sufficiently broad to contain both the given frequency carrier signal and the second frequency carrier signal. Typically, the two RF carrier signals bear an exact integral ratio in frequency, whereby a desired combination of frequencies may not be available for allocation.